The proposed research involves a plasma membrane enzyme, the studies of which were initiated in skeletal muscle, in particular, the skeletal muscle transverse tubular system (TT, T-system, T-tubules). TTs are continuous invaginations of the plasma membrane (sarcolemma or SL) into the interior of the muscle fiber, with the TT surface area being at least 4-fold greater than that of the SL, TTs invaginate towards the interior of the cell and periodically contact the terminal cisternae of the sarcoplasmic reticulum forming junctional complexes known as dyads or triads through the formation of adherens-type junctions. One of the most prominent enzymatic activities associated with highly purified TT vesicles is a calcium- or magnesium-stimulated ATPase (Ca++ or Mg++ - ATPase, EC 3.6.1.3), often simply referred to as the basic ATPase or TT- ATPase. The research examines the structure and function of the hormonally-regulated, lipid second messenger-responsive ATPase. Recent data have conclusively shown that the ATPase in an ecto-enzyme with its catalytic site facing the extracellular milieu. Emphasis has been applied towards the characterization, cloning, and sequencing of a highly immunogenic 85k glycoprotein (85k-GP) in the ATPase complex. Recently the research has evolved from skeletal muscle to other tissues (wherein protein has also been shown to play an important role in other adherens- type junctions), and entirely new insights have been obtained as to the biological function of the ATPase, in particular, its role in cellular recognition and adhesion. The 85k-GP has just been shown in our laboratories to be identical to T-cadherin, a glycophosphoinositide- linked, homophilic, calcium ion-dependent, cellular adhesion protein. The aim of the research is to characterize the components of the ATPase complex and to study its participation in extracellular events. We are particularly interested in exploring our recent data showing that the GPI-anchored is structurally related to the catalytic subunit of the ATPase complex. Studies will be conducted by the AI and the MBRS students to evaluate the role of the enzyme in regulating extracellular adenosine production in cardioventricular tissue and examining particularly significant pharmacological ligands that can modulate the activity of this potent ATPase. The MBRS students will be actively involved in all phases of the research and encouraged to communicate their research findings through posters, abstracts, or journal articles.